Block of ATP‐regulated potassium channels by phentolamine and other α‐adrenoceptor antagonists

Abstract

1 The patch clamp technique has been used to characterize the effects of phentolamine, an unselective blocker of α₁ and α₂-adrenoceptors, on the electrical activity of isolated RINm5F insulin-secreting cells and the gating of ATP-regulated potassium (K⁺_ATP) channels. 2 Current-clamp experiments carried out by use of both conventional whole-cell recordings and nystatin-perforated cells, have demonstrated that phentolamine (5–20 μm) in the complete absence of α-adrenoceptor agonists, caused a sharp depolarization of the cell membrane from approximately −66 mV to −42 mV. This depolarization was associated with the generation of calcium action potential-like spikes. In the continued presence of phentolamine, diazoxide (100 μm) reversed these effects by causing a hyperpolarization of the cell, thereby preventing Ca²⁺ spikes. 3 Unitary current events from K⁺_ATP channels were recorded from both outside-out membrane patches and saponin permeabilized or open-cells. When added to either the inside or the outside of the plasma membrane, phentolamine (0.1–100 μm) blocked openings from these channels. The effects of phentolamine were rapid, sustained and fully reversible. Phentolamine was apparently a more effective blocker of channels from the inside than the outside of the membrane. 4 The K₁ value, corresponding to 50% inhibition of channels was estimated to be approximately 0.7 μm when phentolamine was added to the inside of the membrane and the Hill coefficient approximately 1. 5 Yohimbine (1–10 μm) and the chemically 2-substituted imidazoline α-adrenoceptor antagonists, antazoline (25 μm) and tolazoline (25 μm) were also found to block K⁺_ATP channels in isolated patches of membrane. 6 In conclusion the present study demonstrates that phentolamine and other imidazoline adrenoceptor antagonists have effects upon ATP-sensitive K⁺ channels that are not associated with stimulation of the adrenoceptor.